The invention relates to a method of controlling fuel injection in a fuelling system for a diesel engine. In particular, the invention relates to a method of controlling fuel injection in a common rail diesel system comprising a common rail arranged to deliver fuel to a plurality of fuel injectors.
In conventional common rail fuel systems for diesel engines, it is known to supply a drive current to a fuel injector in order to cause movement of a valve needle away from a valve seating to initiate fuel injection. There is a time delay, referred to as the xe2x80x98injection-start delayxe2x80x99, between the drive current being supplied to the injector and the commencement of movement of the valve needle away from the seating. The injection-start delay is calibrated on test equipment and can be used in an appropriate control algorithm to ensure fuel injection is initiated at the desired stage of the injection cycle.
A problem with this method is that the delay between the supply of the drive current to the injector and the initiation of injection can vary depending on, for example, the pressure of fuel within the common rail, the engine battery voltage and the temperature of fuel. This can have an adverse effect on engine performance.
It is an object of the present invention to provide a control method for a diesel engine fuel system which alleviates this problem.
According to the present invention, a method of controlling the timing of fuel injection in a diesel engine fuel system including a fuel injector supplied with fuel from a high pressure fuel source, comprises the steps of;
varying a drive current which is supplied to the fuel injector at a first time so as to initiate or terminate a first injection of fuel,
monitoring the pressure of fuel within the source so as to detect when a change in fuel pressure occurs,
calculating a time delay between the first time and a second, later time at which a change in fuel pressure within the source is detected, and
using the measured time delay to adjust the time at which the drive current is varied so as to initiate or terminate a subsequent injection of fuel, thereby to ensure initiation or termination of the subsequent injection of fuel occurs at a required time.
By using the time delay measured for a previous injection of fuel to adjust the timing of initiation or termination of injection for a subsequent injection of fuel, any variation in the time delay due to different operating parameters of the engine can be compensated for. For example, the method permits variations in the temperature of fuel, the pressure of fuel within the source, engine battery voltage and injector manufacturing tolerances to be compensated for. This improves injection timing and, hence, engine performance throughout the injector service life.
In one embodiment, the method includes the steps of:
generating a pressure output signal representative of fuel pressure within the source, and
filtering noise frequency components from the pressure output signal and generating a filtered pressure output signal.
The method may comprise the step of increasing the drive current supplied to the fuel injector so as to initiate fuel injection.
In this embodiment, the method may include the step of determining a rate of change of the filtered pressure output signal and comparing said rate of change with a predetermined rate of change of pressure which is indicative of commencement of injection for a given source pressure.
In this embodiment of the invention, the method may comprise the steps of
measuring an injection-start delay between (i) the time at which an increase in the drive current is supplied to the injector so as to initiate injection and (ii) the time at which the rate of change of the filtered rail pressure output signal exceeds the predetermined rate of change of pressure,
and using the measured injection start-delay to adjust the time at which the drive current supplied to the injector is increased to initiate the subsequent injection of fuel.
As the valve needle of the injector commences movement away from its seating to commence injection, the pressure of fuel within the source will decrease. The occurrence of this decrease in fuel pressure can be detected by comparing the rate of change of the filtered pressure output signal with a predetermined rate of change of pressure upon commencement of injection (for a given source pressure) and this is used to provide an indication of the time at which fuel injection is actually commenced following supply of the drive current.
The method may include the steps of estimating an injection-start delay for the primary injection of fuel, calculating the difference between the measured injection-start delay and the estimated injection-start delay and using the difference to adjust the time at which the drive current is increased to initiate the subsequent injection of fuel.
Alternatively, the method may include the step of varying the drive current by decreasing the current supplied to the injector so as to terminate fuel injection.
In one embodiment, the method may include the step of determining a rate of change of the filtered pressure output signal and comparing said rate of change with a predetermined rate of change indicative of termination of injection for a given source pressure.
Conveniently, the method may include the steps of:
measuring an injection-end delay between (i) the time at which decrease in the drive current is supplied to the injector so as to terminate injection and (ii) the time at which the rate of change of the filtered rail pressure output signal exceeds the predetermined rate of change of pressure,
and using the measured injection end-delay to adjust the time at which the drive current supplied to the injector is decreased to terminate a subsequent injection of fuel.
The method may comprise the step of estimating an injection-end delay for the primary injection of fuel, calculating the difference between the measured injection-end delay and the estimated injection-end delay and using the difference to adjust the time at which the drive current is decreased to terminate the subsequent injection of fuel.
The subsequent injection of fuel may immediately follow the primary injection of fuel in an injection cycle.
The method is particularly suitable for use in controlling the injection of fuel in a common rail fuel system comprising a common rail for delivering fuel to a plurality of fuel injectors.
As the source pressure in a diesel engine fuel system is variable, it is necessary to compare the rate of change of the filtered pressure output signal with a predetermined rate of change of pressure for any given source pressure.
Thus, conveniently, the step of comparing said rate of change with a predetermined rate of change is performed in software by reference to a data map or look-up table.